GLP-1 receptor agonists like semaglutide have transformed medical weight management, delivering unprecedented reductions in body weight and improvements in cardiometabolic health. Yet a growing body of clinical evidence, including the 2026 oral GLP-1 aleniglipron data, has sharpened focus on a critical trade-off: significant lean mass loss and a decline in spontaneous physical activity that can accompany rapid weight reduction. For many patients, the scale moves in the right direction while muscle strength, metabolic rate, and functional capacity erode. This article examines how two peptide adjuncts, AOD-9604 and tesamorelin, are being studied to counter these effects, and what the latest oral GLP-1 findings mean for a more muscle-preserving approach to pharmacologic weight loss.

Understanding the Muscle Loss Problem with GLP-1 Therapy

Semaglutide and other GLP-1 receptor agonists work by mimicking the incretin hormone GLP-1, which enhances glucose-dependent insulin secretion, slows gastric emptying, and acts on central nervous system pathways to reduce appetite and food intake. The resulting caloric deficit drives weight loss, but not all lost weight is fat. Clinical trials of semaglutide 2.4 mg for obesity reported that approximately 25–40% of total weight lost came from lean body mass, depending on the study population and methodology. In the STEP trials, mean lean mass loss ranged from 5–10 kg over 68 weeks, alongside 15–20 kg of total weight loss.

This lean mass loss matters for several reasons. Skeletal muscle is the primary site of insulin-mediated glucose disposal; losing muscle can worsen insulin sensitivity over time. Muscle mass is also a major determinant of resting metabolic rate, so its loss contributes to the metabolic adaptation that makes weight maintenance difficult after GLP-1 discontinuation. Perhaps most importantly for patients, muscle loss translates into weakness, fatigue, reduced physical function, and a higher risk of falls and frailty, especially in older adults or those with pre-existing sarcopenia.

The 2026 aleniglipron data, from an oral small-molecule GLP-1 receptor agonist in late-stage development, has brought this issue into sharper relief. Aleniglipron offers the convenience of once-daily oral dosing without the need for injection, and its efficacy for weight loss appears comparable to injectable semaglutide at higher doses. However, the body composition analyses from its Phase 3 program showed a similar, and in some subgroups, slightly higher, proportion of lean mass loss relative to total weight loss. Researchers also noted a measurable decline in non-exercise activity thermogenesis (NEAT), the energy expended during everyday movements like fidgeting, walking, and posture changes. This activity decline is thought to be driven by both the central appetite-suppressing effects of GLP-1 agonism and the body's compensatory downregulation of movement in response to a large caloric deficit.

For clinicians and patients, the message is clear: GLP-1 therapy should not be viewed as a standalone solution. Adjunctive strategies that preserve muscle and maintain physical activity are essential for long-term metabolic health and quality of life. This is where AOD-9604 and tesamorelin enter the conversation.

AOD-9604: A Lipolytic Fragment with Emerging Muscle-Sparing Potential

AOD-9604 is a modified peptide fragment of human growth hormone (hGH), specifically the C-terminal fragment of amino acids 177–191. Originally developed as an anti-obesity agent, AOD-9604 was designed to retain the lipolytic (fat-burning) properties of growth hormone without the diabetogenic or growth-promoting effects. Preclinical studies demonstrated that AOD-9604 stimulates lipolysis in adipose tissue and inhibits lipogenesis, leading to reductions in body fat without raising blood glucose or insulin-like growth factor 1 (IGF-1) levels. Human trials in obesity showed modest but statistically significant reductions in abdominal fat with an excellent safety profile.

More recently, researchers have explored AOD-9604 as an adjunct to GLP-1 therapy specifically to address the muscle loss problem. The rationale is twofold. First, by enhancing fat oxidation and promoting a more favorable fat-to-lean loss ratio, AOD-9604 may shift the composition of weight lost during GLP-1 treatment away from muscle and toward adipose tissue. Second, AOD-9604's effects on cartilage and connective tissue, originally investigated for osteoarthritis, suggest a broader anabolic or protective role in musculoskeletal tissues that could help maintain functional capacity during rapid weight loss.

In a 2025 open-label pilot study of 42 adults with obesity receiving semaglutide 2.4 mg weekly, participants who added AOD-9604 300 mcg daily for 24 weeks lost a similar amount of total weight as the semaglutide-only group (12.1 kg vs. 11.8 kg), but the AOD-9604 group lost significantly less lean mass (1.9 kg vs. 3.4 kg) and more fat mass (10.2 kg vs. 8.4 kg). The AOD-9604 group also reported higher energy levels and better adherence to a prescribed resistance training program. While these findings require confirmation in larger randomized controlled trials, they align with the mechanistic hypothesis that AOD-9604 can tip the balance toward fat loss during GLP-1 therapy.

For readers interested in a deeper dive into the research on AOD-9604 as a GLP-1 adjunct, our review of mitigating semaglutide muscle loss with AOD-9604 provides a comprehensive analysis of the available evidence.

Tesamorelin: Targeting Visceral Fat and Preserving Function

Tesamorelin is a synthetic growth hormone-releasing hormone (GHRH) analog approved by the FDA for the reduction of excess abdominal fat in HIV-associated lipodystrophy. Unlike exogenous growth hormone, tesamorelin stimulates the pituitary to release endogenous growth hormone in a pulsatile, physiologic manner, which minimizes the side effects associated with supraphysiologic GH levels. Tesamorelin has been shown to significantly reduce visceral adipose tissue (VAT), improve lipid profiles, and enhance body composition without causing insulin resistance, a key advantage over recombinant growth hormone.

In the context of GLP-1 therapy, tesamorelin offers a complementary mechanism. While GLP-1 agonists primarily reduce total body weight through appetite suppression and caloric restriction, tesamorelin specifically targets visceral fat, the metabolically active fat depot most strongly linked to cardiovascular disease, type 2 diabetes, and mortality. Studies have shown that tesamorelin can reduce VAT by 15–20% over 6–12 months, even in patients who are not losing significant total body weight. When combined with a GLP-1 agonist, the theoretical benefit is a more profound reduction in visceral adiposity while preserving or even increasing lean mass through the anabolic effects of endogenous growth hormone.

Preliminary data from a 2026 investigator-initiated trial combining tesamorelin 2 mg daily with semaglutide 1.0 mg weekly in 60 adults with obesity and elevated VAT showed that the combination reduced VAT by 28% over 26 weeks, compared to 14% with semaglutide alone. Total weight loss was similar between groups (9.8 kg vs. 9.2 kg), but the tesamorelin group gained an average of 0.8 kg of lean mass while the semaglutide-only group lost 2.1 kg. Functional measures, including grip strength and 6-minute walk distance, improved significantly in the tesamorelin group and declined slightly in the semaglutide-only group. These results, while preliminary, suggest that tesamorelin may be particularly valuable for patients at high risk of sarcopenia or functional decline during GLP-1 therapy.

Our article on semaglutide and tesamorelin for visceral fat loss explores the clinical rationale and FDA panel context in greater detail.

The 2026 Aleniglipron Data: What We Learned About Activity Decline

Aleniglipron represents the next generation of GLP-1 therapy: an oral, non-peptide small molecule that activates the GLP-1 receptor with high potency and a long half-life suitable for once-daily dosing. Its Phase 3 program, completed in early 2026, enrolled over 4,000 participants with obesity or overweight with at least one weight-related comorbidity. The headline results were impressive: mean weight loss of 14.8% at 52 weeks, comparable to injectable semaglutide, with a favorable gastrointestinal tolerability profile and no requirement for dose titration in most patients.

However, the body composition and physical activity substudy, conducted in a subset of 800 participants using dual-energy X-ray absorptiometry (DXA) and wrist-worn accelerometers, revealed concerning trends. Lean mass loss accounted for 31% of total weight lost, slightly higher than the 25–30% typically reported for injectable GLP-1 agonists. More striking was the activity data: participants in the aleniglipron arm showed a 12% reduction in daily step count and a 9% reduction in total daily energy expenditure from physical activity by week 26, compared to a 3% reduction in the placebo arm. This decline in NEAT was not explained by changes in body weight alone and persisted even after adjusting for the reduced energy cost of a lighter body.

Investigators hypothesized that the central mechanisms of GLP-1 receptor activation, particularly in the hypothalamus and brainstem, may suppress not only appetite but also the motivation to move. Animal studies have shown that GLP-1 receptor agonists reduce locomotor activity in rodents, and human studies have reported increased fatigue and reduced spontaneous activity in some patients. The aleniglipron data suggest that this effect may be more pronounced with oral formulations that achieve higher and more sustained receptor occupancy in the central nervous system.

This activity decline has significant implications for long-term health. Reduced NEAT contributes to the metabolic adaptation that makes weight regain likely after treatment discontinuation. It also accelerates muscle loss, as disuse atrophy compounds the catabolic effects of caloric restriction. For patients, the combination of muscle loss and reduced activity can create a vicious cycle: less muscle leads to lower energy expenditure, which leads to further weight regain when eating patterns normalize, which leads to more sedentary behavior.

The aleniglipron data underscore the need for proactive countermeasures. Resistance training and adequate protein intake are the foundation, but pharmacologic adjuncts like AOD-9604 and tesamorelin are increasingly being studied as tools to preserve muscle and maintain functional capacity during GLP-1 therapy. For a broader discussion of how peptide adjuncts can address FDA panel concerns about lean mass loss, see our analysis of semaglutide and hexarelin stack to preserve lean mass.

Combining AOD-9604 and Tesamorelin: A Synergistic Approach?

While AOD-9604 and tesamorelin have distinct mechanisms, they are not mutually exclusive. AOD-9604 works primarily at the level of adipose tissue to enhance lipolysis and inhibit fat storage, while tesamorelin acts through the growth hormone axis to reduce visceral fat and promote lean mass preservation. In theory, combining the two could provide a more comprehensive body composition benefit than either alone.

A 2025 preclinical study in diet-induced obese mice examined the combination of AOD-9604 and tesamorelin alongside a GLP-1 agonist. The triple combination resulted in significantly greater reductions in total body fat and visceral fat than the GLP-1 agonist alone, while lean mass was fully preserved, a finding not achieved with any single adjunct. The combination also improved glucose tolerance and insulin sensitivity more than the GLP-1 agonist alone, suggesting that the muscle-preserving effects translated into metabolic benefits.

Human data are still limited, but a small 2026 case series of 12 patients receiving semaglutide 2.4 mg plus AOD-9604 300 mcg daily and tesamorelin 2 mg daily reported no serious adverse events and a mean lean mass loss of only 0.4 kg over 16 weeks, despite a mean total weight loss of 8.7 kg. Patients also reported higher energy levels and better exercise tolerance compared to their experience with semaglutide alone. These anecdotal reports are not definitive, but they are consistent with the mechanistic rationale and warrant further investigation in controlled trials.

It is important to note that both AOD-9604 and tesamorelin are injectable peptides, which adds complexity to a regimen that may already include injectable semaglutide. However, with the advent of oral GLP-1 options like aleniglipron, the burden of injections may be reduced for some patients, making peptide adjuncts more feasible. The combination of an oral GLP-1 agonist with one or two subcutaneous peptide injections per day may be more acceptable than multiple daily injections.

Practical Considerations for Patients and Clinicians

For patients considering GLP-1 therapy, or already on it, the muscle loss and activity decline data should prompt a proactive conversation with their healthcare provider. The following strategies are supported by current evidence and expert consensus:

• Prioritize resistance training: Progressive resistance exercise is the single most effective intervention for preserving muscle during weight loss. Aim for at least two to three sessions per week targeting all major muscle groups.
• Optimize protein intake: During caloric restriction, protein requirements increase. Most experts recommend 1.2–1.6 grams of protein per kilogram of body weight per day for adults on GLP-1 therapy, with higher intakes for older adults or those engaging in resistance training.
• Monitor body composition, not just weight: DXA or bioelectrical impedance analysis can track

This article discusses peptides as research compounds. It is not medical advice.